Siddharth Bhat

16 Mar 2021

â€¢

7 min read

In this blog post, we will learn about `Contravariant`

and
`Divisible`

which provide duals for `Data.Functor`

and `Data.Applicative`

respectively.

```
{-# LANGUAGE NoImplicitPrelude #-}
{-# LANGUAGE InstanceSigs #-}
import GHC.Base hiding (Functor)
import GHC.Float -- for Ord instance of Float
import GHC.Show -- for show
```

First, a quick recap of functors:

```
class Functor f where
fmap :: (a -> b) -> f a -> f b
```

This lets us lift a function `f: a -> b`

into a `fmap f: f a -> f b`

.
The dual is called `Contravariant`

:

```
class Contravariant f where
contramap :: (a -> b) -> f b -> f a
```

Let us look at some example to build our intuition of such a typeclass.

The classic example is that of a *predicate*, which is something that
tells us whether a value of type `t`

obeys some property or not:

```
data Predicate t = Predicate { runPredicate :: t -> Bool }
instance Contravariant Predicate where
contramap :: (a -> b)
-> Predicate b -- b -> Bool
-> Predicate a -- a -> Bool
contramap a2b (Predicate b2bool) =
Predicate (\a -> b2bool (a2b a))
```

An example of such a thing is if we know how to check a real number is greater than zero:

```
reGt0 :: Predicate Float
reGt0 = Predicate (\x -> x > 0.0)
```

and we can converts integers into reals:

```
intToReal :: Int -> Float
intToReal i = error "TODO" -- fromIntegral
```

then we can check if an integer is greater than zero:

```
intGt0 :: Predicate Int
intGt0 = contramap intToReal reGt0
```

This is described by the picture:

So, such a `Predicate Float`

"consumes" a `Float`

to produce a `Bool`

.
We can pull back the consumption along a function `Int -> Float`

to consume
a `Int`

and produce a `Bool`

.

```
class Functor f => Applicative f where
pure :: a -> f a
(<*>) :: f (a -> b) -> f a -> f b
```

Recall that an `Applicative`

allow us to work with
tuples:

```
liftA2 :: (a -> b -> c) -> f a -> f b -> f c
```

We can write the type of liftA2 to be more suggestive as:

```
liftA2 :: ((a, b) -> c) -> ((f a, f b) -> f c)
```

If we can **combine** a tuple `(a, b)`

into a value `c`

,
then we can glue lifted values `(f a, f b)`

into a lifted `f c`

.

The dual, called `Divisible`

, says that if we can **break** a value `c`

into `(a, b)`

,
then we can glue lifted values `(f a, f b)`

into a lifted `f c`

.

```
class Contravariant f => Divisible f where
divide :: (c -> (a, b)) -> f a -> f b -> f c
conquer :: f a
```

The `conquer`

is some sort of "default procedure" we can perform for
any value. It'll be something benign, as we'll see when we check out the examples.

Above, we have a picture of how to think about `Divisible`

. The box with pink-and-blue is a `c`

, that contains an `a`

and a `b`

. We have a function `p`

that shows us how to split a `c`

into an `a`

and a `b`

. We also have `f a`

and `f b`

, which consume `a, b`

to produce some orange output. If we have this data, we can build an `f c`

, something that can consume a `c`

, by (1) splitting `c`

into `(a, b)`

, and then consuming the `(a, b)`

using `f a`

and `f b`

.

We can continue our example of predicates. If we know how to check if
something holds for `a`

and something holds for `b`

, we can check how something
holds for `(a, b)`

: check for **both** `a`

**and** `b`

. So, this would be:

```
instance Divisible Predicate where
divide :: (c -> (a, b)) ->
Predicate a -> Predicate b -> Predicate c
divide c2ab (Predicate a2bool) (Predicate b2bool) =
Predicate (\c -> let (a, b) = c2ab c
in a2bool a && b2bool b)
```

As for when we know nothing, we could either allow it or disallow it.
In this case, since we are `&&`

ing information, the way to be "benign"
is to allow things (that is, return a `True`

). Since `True && b = b`

,
we are sure that the `conquer`

is indeed benign.

```
conquer :: Predicate a
conquer = Predicate (\a -> True)
```

Consider the ability to convert a data type to a string. These "consume" the (varying)
data types to produce a `String`

. So, for example:

```
data Serializer a = Serializer { serialize :: a -> String }
```

If we know how to print an `b`

(that is, we have `b2string :: b-> String`

),
and we can turn `a`

's into `b`

s, we compose the two to print `a`

s:

```
instance Contravariant Serializer where
contramap :: (a -> b) -> Serializer b -> Serializer a
contramap a2b (Serializer b2string) =
Serializer (\a -> b2string (a2b a))
```

For our `Divisible`

instance, if we can print `a`

and `b`

, and we
can break a `c`

into an `(a, b)`

, we (1) break the `c`

down, and then
(2) print the `a`

and the `b`

, and (3) concatenate the string representation
of `a`

and `b`

:

```
instance Divisible Serializer where
divide :: (c -> (a, b))
-> Serializer a
-> Serializer b
-> Serializer c
divide c2ab (Serializer a2str) (Serializer b2str) =
Serializer (\c -> let (a, b) = c2ab c
in (a2str a) <> (b2str b))
```

As for `conquer`

, if we don't know how to print something, the best thing
to do is to not print anything at all. This prevents us from garbling output.
Thus, the benign choice for `conquer`

is to print an empty string:

```
conquer :: Serializer a
conquer = Serializer (\a -> "")
```

We can put `Serializer`

work immediately. For example, say we know
how to serializer `Int`

s and `Float`

s:

```
intSerial :: Serializer Int
intSerial = Serializer (\i -> show i)
floatSerial :: Serializer Float
floatSerial = Serializer (\f -> show f)
```

If we now have a type that contains `Int`

and `Float`

, no problem! `Divisible`

has our back to combine the `Serializer`

s together:

```
data Foo = Foo Int Float
fooSerial :: Serializer Foo
fooSerial = divide (\(Foo i f) -> (i, f))
intSerial floatSerial
```

We can generalize both examples: we have seen before:
`Predicate`

is all functions into a fixed output
type `Bool`

, while `Serializer`

is functions into a fixed output type `String`

. We need
to know how to combine the outputs --- in the case of `Bool`

, we combined the outputs
with `&&`

. In the case of `String`

, we combined the outputs with `<>`

. In general,
we need a **monoid**.

```
data Into y x = Into { runInto :: x -> y }
instance Contravariant (Into y) where
contramap :: (b -> a)
-> Into y a -- a -> y
-> Into y b -- b -> y
contramap b2a (Into a2y) =
Into (\b -> a2y (b2a b))
```

For the `divide`

, we combine the data from `a`

and `b`

using the
monoid of `y`

:

```
instance Monoid y => Divisible (Into y) where
divide :: (c -> (a, b))
-> Into y a -- a -> y
-> Into y b -- b -> y
-> Into y c -- c -> y
divide c2ab (Into a2y) (Into b2y) =
Into (\c -> let (a, b) = c2ab c
in (a2y a) <> (b2y b))
```

For conquer, the "benign instance" is the `mempty`

value of the monoid,
which by definition does not "interact" with any element, as
`mempty <> m = m`

and `m <> mempty = m`

:

```
conquer :: Into y a -- a -> y
conquer = Into (\a -> mempty)
```

In all of these examples, we have (a) A data structure that can be *decomposed*: this is
the part of `c -> (a, b)`

, and (b) A consumer of data: `f a`

is "something that can consume an `a`

.

So far, I have been skating on intuition, without telling you what the *laws* `Divisible`

must follow
are. Let's get formal now. For a given `Contravariant f`

, we need a `fmap`

-like law to hold:

`fmap`

's law:: `fmap (f . g) = fmap f . fmap g`contramap`

's law:`contramap (f . g) = contramap g . contramap f`

See that the order gets flipped in comparison to `fmap`

. Let us check that
this law holds for `Into y`

, since that was the most general example.

```
contramap :: (p -> q) -> Into y q -> Into y p
x2q :: x -> q
contramap (x2q . p2x) $ (Into q2y) =?=
contramap p2x . conramap x2q $ (Into q2y)
```

We can rewrite our `Into y`

definition to be easier to manipulate using
point-free style:

```
instance Contravariant Into where
contramap :: (b -> a)
-> Into a -- a -> y
-> Into b -- b -> y
contramap b2a (Into a2y) = Into (a2y . b2a) -- b -> y
```

if we now try to simplify:

`contramap p2x . contramap x2q $ (Into q2y)`

- Remove
`.`

and`$`

:`contramap p2x (contramap x2q (Into q2y))`

- unwrap inner
`contramap`

: `contramap p2x (Into (q2y . x2q)) - unwrap outer
`contramap`

:`Into (q2y . x2q . p2x)`

- re-group
`.`

:`contramap`

:`Into (q2y . (x2q . p2x))`

- introduce back
`contramap`

:`contramap (x2q . p2x) (Into q2y)`

thus we are done! We've shown that the `Contravariant`

laws hold for `Into`

The laws follow from some [category theory](https://hackage.haskell.org/package/contravariant-1.5.3/docs/Data-Functor-Contravariant-Divisible.html# g:3). We need that for the function:

```
delta :: a -> (a, a)
delta a = (a, a)
```

the following relations between `divide`

and `conquer`

hold:

- First, let us think about
`divide delta`

. It means that we perform the*same*action on the left element and right element of the tuple since our tuple is built from the same element`a`

.

```
dd :: Divisible f => f a -> f a -> f a
dd = divide delta
```
1. `conquer` is an identity element for `divide delta`:
```hs
dd m conquer = dd conquer m
```
2. `divide delta` is associative:
```hs
dd m (dd n o) = dd (dd m n) o
```
So this is saying that `divide delta` is monoidal, with `conquer`
as the identity element. Let's verify what happens in our case of `Into y`.
0. Let `a2y, a2y' :: Into y a`.
1. Expand the definition: `dd (a2y, a2y') = divide delta (a2y, a2y')`
2. Expand `divide`:
```hs
divide delta a2y, a2y'
= Into (\c -> let (a, b) = delta c
in (a2y a) <> (a2y' b))
```
3. Substitue `delta c = (c, c)`
```hs
divide delta a2y, a2y'
= Into (\c -> let (a, b) = (c, c)
in (a2y a) <> (a2y' b))
```
4. Replace `a, b` with `c`
```
divide delta a2y, a2y' = Into (\c -> (a2y c) <> (a2y' c))
```
Great, so we have a simple enough definition of what `dd` does; It runs
both `a2y` and `a2y'` ou the same input and smashes the results. At this
point, it should be hopefully _somewhat_ clear why the laws hold for `Into`:
1. We build `conquer` using `mempty`, Since `mempty` is the identity for `(<>)`,
`conquer should be the identity for `divide delta`.
2. We are smashing outputs using `(<>)` in `divide delta`. As `(<>)` is associative, we should get
associativity of `divide delta` for free.
![divide-delta-conquer.png](https://functionalworks\-backend\-\-prod.s3.amazonaws.com/logos/51e8bfabb350afa67eec66d52ea2b4ac)
Pictorially, we are combining two machines: one that turns `x2y`, and one that is `conquer` which is "useless". Since we start by copying `x` using `delta x = (x, x)`, whatever `conquer` does is useless, and then only effect that's leftover is whatever the `x2y` does. So we can simplify the above figure by eliminating the bottom part of the computation, leaving us with this:
![divide-delta-conquer-simplify.png](https://functionalworks\-backend\-\-prod.s3.amazonaws.com/logos/bf5f27875bd4848699b6b28c07d20250)
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